The present invention relates to an apparatus fort melting metals and a method for melting metals, more specifically to an apparatus for melting scraps, ingots, etc. of iron, copper, aluminum, etc. using an oxygen fuel burner to which oxygen is supplied as a combustion assisting gas.
There are known metal melting furnaces in which fossil fuels are burned using oxygen fuel burners to which oxygen is supplied as a combustion assisting gas, and scraps or ingots of iron, copper, aluminum, etc. are melted by the heat of combustion. Metal melting furnaces utilizing such oxygen fuel burners are described, for example, in Japanese Unexamined PCT Publication No. 501810/1981 and Japanese Unexamined Patent Publication Nos. 215919/1989, 93012/1990, 271804/1993 and 271807/1993.
These metal melting furnaces generally are each provided with a melting section where a metallic raw material is melted using oxygen fuel burners and a preheating section where the metallic raw material is preheated. In the metal melting furnaces described in Japanese Unexamined PCT Publication No. 501810/1981 and Japanese Unexamined Patent Publication No. 215919/1989, the preheating section is located above the melting section via a closing grid so as to preheat a next charge of metallic raw material. However, in the metal melting furnace having such iron grid above the melting section, the iron grid is exposed to high temperature, so that it must be cooled with water and the like, causing not only a great heat loss but also water leakage, troubles in opening and closing the iron grid, etc. due to severe environment to which the melting furnace is exposed.
Meanwhile, in the metal melting furnace described in Japanese Unexamined Patent Publication No. 271807/1993, which is a so-called reverberatory furnace, a metallic raw material is introduced gravitationally through a slant passage defined in the wall of the furnace into the melting section while it is preheated by the discharge gas from the melting section when the metallic raw material passes through the slant passage. In this case, however, the hot discharge gas tends to flow the upper space of the slant passage serving as the preheating section, so that it is difficult to preheat fully the metallic raw material falling along the lower part of the slant passage, and it is also difficult to control the falling speed of the metallic raw material, because the material is introduced by free fall.
Generally, in a metal melting furnace integrated with the preheating section where the metallic raw material is preheated, the rate of introducing the metallic raw material from the preheating section into the melting section significantly influences the heat efficiency. More specifically, the metallic raw material is preferably introduced at the same rate as it is melted in the melting section. If the raw material introducing rate is too high, a mixture of an unmelted metal portion and a molten metal portion dwells at the bottom of the melting section, and further there may occur a phenomenon that the molten metal solidifies due to heat loss from the bottom of the furnace. On the other hand, if the introducing rate is too low, it takes much time for introducing the metallic raw material to consume extra energy.
Further, metal melting furnaces utilizing oxygen fuel burners can increase heat efficiency to 50% or more. Although they have excellent efficiency as metal melting furnaces, they consume large amounts of oxygen, and the overall energy consumption is great when electric energy necessary for producing oxygen is taken into consideration. For example, when 1 ton of iron is melted, about 120 Nm3 of oxygen is consumed. Thus, an electric power of about 0.45 kW is consumed for producing 1 Nm3 of oxygen in the form of high-purity oxygen (oxygen content:  greater than 99%) using an air liquefying and separating unit, so that a total electric power of about 55 kW is necessary for melting one ton of iron.
Therefore, it is an objective of the present invention to provide an apparatus for melting metals which can control the rate of introducing a metallic raw material from the preheating section to the melting section to be within an optimum range and can achieve efficient melting of the metallic raw material with oxygen fuel burners only, and which enables economical supply of oxygen serving as a combustion assisting gas to the oxygen fuel burners and can achieve reduction in the total cost of melting metals, as well as, to provide a method for melting metals.
The apparatus for melting a metal according to the present invention contains a metal melting furnace for melting a metallic raw material with a flame of an oxygen fuel burner to which oxygen is supplied as a combustion assisting gas, and an oxygen supply source for supplying oxygen as the combustion assisting gas to the oxygen fuel burner. The metal melting furnace has a preheating section for preheating the metallic raw material above a melting section to which the oxygen fuel burner is attached and a reduced section, located between the melting section and the preheating section, having an inside diameter smaller than those of the melting section and preheating section. According to a first aspect of the present invention, the oxygen supply source is a pressure swing adsorption separator employing an adsorbent which adsorbs preferentially atmospheric nitrogen and supplying a low-purity oxygen having an oxygen content of 65 to 94% to the oxygen fuel burner. According to a second aspect of the present invention, the oxygen supply source is an air liquefying and separating unit which condenses air to fractionate oxygen and supplies a low-purity oxygen having an oxygen content of 65 to 99% to the oxygen fuel burner. According to a third aspect of the present invention, the oxygen supply source is an oxygen-air mixer which mixes a low-purity or high-purity oxygen with air and supplies a low-purity oxygen having an oxygen content of 65 to 99% to the oxygen fuel burner.
According to the method for melting a metallic material of the present invention, a metallic raw material is melted with a flame of an oxygen fuel burner to which oxygen is supplied as a combustion assisting gas from an oxygen supply source. This method employs a metal melting furnace having a preheating section for preheating the metallic raw material above a melting section to which the oxygen fuel burner is attached and a reduced section, located between the melting section and the preheating section, having an inside diameter smaller than those of the melting section and preheating section and also employs a low-purity oxygen having an oxygen content of 65 to 99% as the combustion assisting gas.
The oxygen fuel burners employable according to this invention are those which form high-temperature flames by burning fossil fuels such as heavy oil, kerosene, pulverized coal, propane gas and natural gases employing low-purity oxygen as a combustion assisting gas. As the oxygen fuel burner, for example, those disclosed in Japanese Patent Publication Nos. 3122/1991 and 43096/1995 may be employed. However, this invention is not to be limited to these burners, but burners of various structures may be employed depending on the kind of fuel and the like.
According to the apparatus and method for melting metals of the present invention, by employing a metal melting furnace in which the preheating section is provided via the reduced section above the melting section, not only the metallic raw material can be preheated efficiently, but also the amount of metallic raw material falling from the preheating section into the melting section can be controlled to an optimum level. Thus, there is no need of incorporating a device for controlling the charge of raw material such as the conventional iron grid, and, for example, scraps or ingots of iron, copper, aluminum, etc. can be melted efficiently in the melting furnace having such simple structure, thus achieving reduction in the production cost and maintenance cost, as well as, improvement of heat efficiency and reduction of melting time.
Besides, the cost required for producing oxygen can be reduced by employing a low-purity oxygen having an oxygen content of 65 to 99% as a combustion assisting gas for the oxygen fuel burners in the metal melting furnace, leading to great reduction in the total metal melting cost.